I have finally completed the campaign with 4 players (I have no friends that play Gears and just needed to complete the prologue with 4 players for ages but could never get 4 players to join before the round ended). It shows that I've completed every chapter with 4 player co-op but the mutator has not unlocked (I think it's friendly fire or something). If there is an achievement for it I don't think this has unlocked either. Any advice or help to sort this would be appreciated. It's not a major issue but is a bit annoying when it takes a long time to achieve something and then it doesn't give you the unlocks etc.